1. Field of the Invention
The present invention relates to an apparatus for controlling an inverter circuit of an induction heat cooker, and more particularly to an apparatus for controlling an inverter circuit of an induction heat cooker capable of generating a driving pulse whose width is varied in response to an input voltage to reduce a voltage difference between both ends of a switch to drive a switching operation of the inverter circuit, and, simultaneously, controlling the inverter circuit so that a turn-off time of the driving pulse is varied according to separation state of the cooking container and a cooking state of food.
2. Description of the Related Art
A cooker such as a rice cooker, an electric pan, a slow cooker, an electric kettle and the like is a device cooking food included in a container thereof by heating the food above a predetermined temperature.
Generally, a cooker includes a body having a PCB (Printed Circuit Board) to operate and determine whether power is applied thereto in response to a user's button operation, a cooking container for containing food to be placed therein, and a heater installed under the cooking container or in the body for heating food.
This specification will be described in respect to an induction heat cooker including coils, each of which is regularly formed in a predetermined part to be put a cooking container, and cooking food in the cooking container made of a magnetic material heated by eddy currents caused by magnetic fields as current flows in the coils.
Referring to FIG. 1, a prior art inverter circuit of an induction heat cooker will be described in detail below.
An inverter circuit 41 of an induction heat cooker switches a switch element to generate a high frequency current with relatively high power and to heat a cooking container including food by induction heat. Such an inverter circuit 41 is switched by a control signal to supply current to coils, thereby supplying heat to the cooking container. The construction of the prior art inverter circuit will be described in detail below.
The inverter circuit 41 includes an AC power source 10 supplying an AC power source to each element, a rectifier 20 for rectifying the AC power source, a filter 30 for filtering the AC power rectified in the rectifier 20 to output a filtered AC power, and a switching unit 40 inputting the filtered AC power and applying a high power to the coils in response to a switching operation.
Also, an inverter circuit controller 81 controlling the inverter circuit 41 includes an input voltage detector 50 for detecting a variation of voltage inputted to the inverter circuit 41 connected to the AC power source 10, a pulse width variation controller 60 varying a width of a driving pulse driving the switching unit 40 in response to a variation of the input voltage, and a gate drive unit 80 for transmitting the driving pulse generated from the pulse width variation controller 60 to the switching unit 40 to perform the switching operation.
Such a pulse width variation controller 60 includes a differential amplifier 61 generating a control signal for varying a width of high level interval of the driving pulse in response to a variation from the input voltage detector 50, and a pulse generation IC (Integrated Chip) 62 for determining a turn-off time of the driving pulse.
Therefore, the width of the driving pulse for driving the switching unit 40 of the inverter circuit 41 is varied such that the width of high level interval of the driving pulse is decreased in a relatively high input voltage portion and the width of high level interval thereof is increased in a relatively low input voltage portion, therefore a voltage increase at both ends of the switch can be repressed when the inverter circuit is driven.
Here, since a turn-off time of the driving pulse is determined by resisters and capacitors each of which has respective values in the inverter circuit, it can be maintained constantly even when heating loads are varied in response to variations of separation state of the cooking container or cooking state of food therein.
FIG. 2 is views illustrating waveforms of a switch voltage and a driving pulse of a prior art inverter circuit in response to variations of heat load. With reference to FIG. 2, the prior art problems will be described in detail below.
A waveform of G1 drawn by a bold line indicates a state that a cooking container is placed to the cooker. Namely, the waveform of G1 is a graph showing that the cooker has normal heat loads as the coils normally contact the cooking container. Also, a waveform of G2 drawn by a dotted line indicates a state that the cooking container is separated from the cooker. Namely, the waveform of G2 is a graph showing that the cooker has no heat load.
The reason why the waveforms G1 an G2 are different is that the cooking container and food, which are referred as heat load, are gradually heated as current flows in the coils, such that the heat load and magnetic characteristics of the coils vary, and thus characteristics of switch voltages differ.
Namely, the heated load of the induction heat cooker is varied according to separation of the cooking container, state variation of heated food, material and deformation of the cooking container, etc. The variation of the heated load causes a resonant inductance value of the coils.
Especially, the resonant inductance value in a state with no heat load, wherein the cooking container is separated, is much greater than that of the inverter circuit, which is previously set. Therefore, a resonant time of the switch element increases.
As such, although the resonant time is varied in response to the variation of the inductance value, the turn-off time of the driving pulse in the prior art inverter circuit is fixedly set when it is manufactured. Therefore, the prior art inverter circuit has disadvantages in that the switch element has a high voltage when it is turned on, if the resonant inductance value is increased.
When the cooking container is separated, or in a no heat load state, if a resonant time is increased by an increased resonant inductance value, the switch element is set to a relatively high switch voltage while it does not secure a relatively sufficient turn off time, and a relatively large short current flows through the switch element. Therefore the switch element is damaged. Accordingly, the damage of the switch element causes a breakdown of the induction heat cooker and burdens a user with costs for repairing the breakdown thereof. Also, they deteriorate the endurance of the cooker.